Conventionally, there has been suggested an image processing method for recognizing a lane mark such as a white line on the basis of a road image ahead of a vehicle captured by an imaging device mounted on the vehicle (for example, refer to Japanese Patent Laid-Open No. Hei 11 (1999)-147473 and Japanese Patent Laid-Open No. Hei 11 (1999)-147481). The recognition result of the lane mark is used, for example, in order to control the lateral position of the vehicle with reference to the position of the lane marks which lie on either side of the vehicle. According to the conventional technology, the white lines (or their edges) are recognized by using a property that the luminance of the white lines contained in the road image is higher than that of other portions of the road.